Higher efficiency of command or date transfer, sharing of information resources, and sharing of hardware resources can be accomplished by connecting a plurality of devices to form a network. Further, in recent years, wireless networks are attracting attention as systems that free users from interconnections according to cable systems.
Examples of standard specifications concerning wireless networks include IEEE (The Institute of Electrical and Electronics Engineers) 802.11, HiperLAN/2, IEEE 802.15.3, and Bluetooth communication. In recent years, wireless LAN systems have become inexpensive and PCs have begun to come equipped with a wireless LAN system, resulting in remarkable widespread use of wireless LAN.
Relatively small-scaled wireless communication systems are utilized in homes or the like for data transfer between a host device and a terminal device. Examples of such a host device include stationary household electrical appliances such as a television, a monitor, a printer, a PC, a VTR, and a DVD player. Examples of the terminal device include portable devices in which power consumption is desired to be reduced as much as possible, such as a digital camera, a video camera, a cellular phone, a portable information terminal, and a portable music player. One example of the applications of this type of system is uploading image data picked up by a camera-equipped cellular phone or a digital camera to a PC via a wireless LAN.
By way of example, there has been proposed a digital camera that is advantageous when performing Bluetooth communication favorably (see, for example, Patent Document 1). Specifically, the digital camera includes a case and a shutter button provided on the case, and the shutter button is provided at a left or right edge portion of the upper part of the case. In addition, an antenna for Bluetooth communication is provided inside the case and at the opposite (left or right) edge portion of the upper part of the case.
However, the wireless LAN was originally designed and developed on the assumption that it would be used in computers, and when attached to a portable device, it suffers a problem of power consumption. Many of the wireless LAN cards (IEEE 802.11b) currently on the market have a power consumption of or more than 800 mW (at the time of transmission) and of or more than 600 mW (at the time of reception). This power consumption is a heavy burden to portable devices, which are powered by butteries.
Restricting the use of the wireless LAN capability to short range operation in order to reduce transmission power results only in an approximately 80 percent reduction in power consumption. In particular, transfer from an image input device, such as a digital camera, to an image display device takes the form of communication in which transmission occupies almost all parts of communication, which constitutes an additional reason that a wireless transfer means with low power consumption is desired.
As for the Bluetooth communication, because its transfer rate is low, a maximum of 720 kbps, it is inconvenient for transfer of today's images with improved quality and increased file size as it takes much time.
In contrast, the use of radio transmission employing a reflected wave based on the backscatter system as used in RFID realizes low power consumption in the communication form in which transmission occupies almost all parts of communication between devices, for example.
A wireless communication system based on the backscatter system is constructed of a reflector that transmits data by means of a modulated reflected wave and a reflected wave reader that retrieves the data from the reflected wave from the reflector. At the time of data transfer, the reflected wave reader transmits an unmodulated carrier. On the other hand, the reflector uses, for example, a load impedance operation, such as On/Off of termination of an antenna, to modulate the unmodulated carrier in accordance with data to be transferred, thereby sending the data. Then, the reflected wave reader receives the reflected wave and performs demodulation/decoding thereon, thereby acquiring the transmitted data.
The reflector is constructed of, for example, an antenna for causing an incoming continuous radio wave to be reflected, a circuit for generating transmission data, and an impedance changing circuit for changing the impedance of the antenna in accordance with the transmission data (see, for example, Patent Document 2).
In the reflected wave transmission system, an antenna switch for changing the load impedance of the antenna (i.e., modulating the reflected wave) is generally formed by a gallium arsenide IC, and its power consumption is at or lower than tens of microwatts. As for the average power at the time of data transfer, data transfer is possible with or less than 10 mW in the case of a delivery confirmation system and with tens of microwatts in the case of a unidirectional transfer. This shows an overwhelming superiority in performance as compared with the average power consumptions of common wireless LANs (see, for example, Japanese Patent Application No. 2003-291809). Therefore, even in the case where a terminal device for information storage is mounted on a battery-powered portable device such as a digital camera, significant prolongation of the battery life can be achieved by saving the power consumption at the time of data transfer.
In the reflected wave transmission system, which is a low power consumption wireless communication system and which employs a reflected wave, the reflected wave reader needs to be transmitting the unmodulated carrier when data is transmitted from a tag. Therefore, the unmodulated carrier transmitted from the reflected wave reader may cause a problem of interference with other wireless communication systems. Further, in order to receive the reflected wave with sufficient reception intensity, the reflected wave reader needs to transmit the unmodulated carrier with a relatively large power, resulting in a large power consumption during transmission of the unmodulated carrier. Still further, in the case where the 2.4 GHz band, which falls within the ISM (Industry Science Medical) band, is utilized for the reflected wave transmission system, interference with other adjacent radio standards, such as IEEE 802.11b/g and Bluetooth, may occur, resulting in communication interference with each other.
A pedestal-like connection stand, called a “cradle”, on which a device is to be placed is sometimes used for a portable device, such as a digital camera or a PDA (Personal Digital Assistant), that does not have a means of direct connection to a PC. The cradle has a point of electrical connection with a portable device mounted thereon and a means of direct connection to a PC, which enables data exchange between the two devices (see, for example, Patent Document 3). In general, the cradle and the PC are serially connected with each other via a USB (Universal Serial Bus) cable or the like. Many cradles have the capability to charge the battery of a portable device as well. Placing the cradle at a position close to the PC results in an increased affinity between the portable device and the PC, and the convenience of the portable device is expected to be improved by the intermediacy of the cradle.
However, in the case where the point of connection between the cradle and the portable device is of the contact type, the point of connection suffers a maintenance problem. Further, in the case of a digital camera, operation of the digital camera, such as photographing, is cumbersome while it is placed on the cradle. In other words, the digital camera might decrease in the ease of use depending on its design.
[Patent Document 1]
Japanese Patent Laid-open No. 2004-56711
[Patent Document 2]
Japanese Patent Laid-open No. Hei 01-182782
[Patent Document 3]
Japanese Patent Laid-open No. 2004-135119